Cyclin-dependent kinase 5 (Cdk5) is predominantly expressed in the nervous system. Its activity is primarily restricted neuronal cell due to its association with neuron specific molecules p35 and p39.It is a multifunctional kinase, involved in neuronal migration, synaptic transmission, and survival. Cdk5 targetes proteins from neuronal differentiation to synaptic function, is tightly regulated when complexed with p35, its co-activator. It is one of several kinases that phosphorylate neurofilaments, tau and large number of other synaptic proteins. Its diverse roles stem, in part, from its cross-talk interactions with other kinases in signal transduction networks underlying neuronal cell survival, growth and differentiation. We have shown, for example, that Cdk5 down regulates MAPKs and JNKs and up regulates PI3Ks. These results suggest that Cdk5 normally modulates the intensity of response of other kinases to specific signals underlying neuronal survival. Its multifunctional role at the synapse is complex and probably involves other novel substrates. Normally, Cdk5 activity is tightly regulated but under conditions of neuronal stress it is deregulated leading to hyperactivity, neuronal pathology and cell death. Accordingly, Cdk5 has been implicated in certain neurodegenerative disorders such as Alzheimer's Disease (AD). A model of Cdk5s role in neurodegeneration suggests that a stress-induced influx of calcium ions into neurons activates calpain, a Ca++- activated protease, which cleaves p35 into p25 and a p10 fragment. p25, in turn, forms a more stable Cdk5/p25 hyperactive complex, that hyperphosphorylates tau and other neuronal cytoskeletal proteins, and induces cell death. Indeed, increased levels of p25 and Cdk5 activity have been reported in AD brains. That p25 may be toxic comes from studies of cortical neurons treated with Abeta-amyloid peptide,a key marker of AD pathology, where p35 is converted to p25 accompanied by hyper-activated Cdk5, tau and neurofilament hyperphosphorylation and apoptosis. Expression of the Cdk5/p25 complex seems to be primarily responsible for the tau and neurofilament pathology and suggests that a therapeutic approach directed specifically at this target might prove successful. For most of these studies, however, the focus has been amon various laboratories around world on aminothiazol compounds resembling roscovitine, a kinase inhibitor that ccompetes with the ATP binding site in Cdk5 and other kinases. These drugs do not act specifically on Cdk5/p25 but also inhibit Cdk5/p35 and other kinases essential for normal development and function. This could be responsible for serious secondary side effects and thereby compromise any therapeutic value. Our approach to this problem, however, is based on current studies where we identified a small peptide of 24 amino acid (aa) residues of p35 that inhibited Cdk5/p25 activity and rescued cortical neurons from induced apoptosis without affecting Cdk5/p35 activity. This approach might prove to be a more effective way to suppress deregulated Cdk5/p25 hyperactivity inducing neurodegenerative pathology. The small size and specificity of p5 inhibition make it an excellent candidate for therapeutic trials in animal models of AD and other neurodegenerative disorders associated with Cdk5 deregulation. This may provide a possible new and novel therapeutic route for intervention to prevent or reduce the neurodegenerative pathology induced by Cdk5 deregulation. Most importantly, our studies show that p5 is a most effective inhibitor that inhibits Cdk5/p25 hyperactivity without affecting the activity of the endogenous Cdk5/p35 activity, in neurons, nor the activity of related cell cycle Cdks. This suggests that as a therapeutic agent p5 might have minimal side effects and hence, might be a prime therapeutic candidate for neurodegenerative disorders evoked by hyperactive Cdk5/p25. Due to their size and composition, peptides and proteins do not readily enter cells. However, addition of the small (9-12mer) basic amino acid-containing peptide derived from the HIV-1 TAT protein, termed a protein transduction domain (PTD), results in rapid entrance of proteins into cells in culture. Therefore, for our studies we modified p5 by conjugating it with TAT PTD at the C-terminal end and with FITC at the N-terminus to produce a fluorescent TFP5 peptide Initially, we studied the effect of TFP5 on Cdk5/p35 and Cdk5/p25 activity in test tube experiments;like p5, it inhibited both.To study the inhibitory effects of TFP5 in neurons, initially cortical neurons were incubated in the presence of TFP5 (0.05uM) for 48 hrs. TFP5 entered and localized in the cells without any toxicity. Next cortical neuronal cultures were transfected with or without p25 and treated with TFP5. Cdk5/p35 and cdk5/p25 activities were measured using histone as a substrate.TFP5 specifically inhibits the Cdk5/p25 but not Cdk5/p35 activity. Encouraged by these results, we injected (ip)TFP5 in adult mice and found that TFP5 not only permeates to peripheral organs but also passes the blood brain barrier (BBB) and localizes in different parts of the brain without any toxicity. A truncated peptide from p35, a Cdk5 activator, prevents Alzheimers disease phenotypes in model mice for publication in the FASEB journal as a research communication. The findings are novel and we believe are of interest to a vast majority of readers in public health, particularly those with interests in Alzheimers Disease (AD) and other neurodegenerative disorders. Currently there is no specific therapeutic drug available for AD. Here, we report on the identity of a novel peptide showing rescue of AD phenotypes in an 5XFAD model mouse. A wide range of AD pathological and behavioral deficits are dramatically reduced within a few days after peptide injection. These include reduction in amyloid plaques, tau tangles, microglial and astrocytic inflammatory effects and neuronal cell death. Longevity is also extended by two months in the AD mouse model. No toxicity is observed in normal control animals even at a high dosage of 200mg/kg of the peptide. Animals exhibit normal weight gains and behavior over an extended period up to 2 yrs. The rescue of abnormal behavior occurs within a few days post injection. Tests of working memory (Y-maze), motor control (rotorod) and anxiety behavior showed significant improvement after injection. The data are consistent with the hypothesis that hyperactivated Cdk5/p25 plays a major role in the complex etiology of AD.